...So Ran is gonna shrink in size when she gets older and starts losing tails? In any case, seeing an adult Chen is nice.

Ran will undergo anime old person chibification, and will lose details like tails and fingers.

Late comment is late but...
Actually, that's canon for Humanoid Fox-Spirits in Japanese folklore, so yes, if ZUN (or author of whatever doujin the context is placed in) decides to use semi-traditional folklore as the base then Ran will lose tails and shrink.
She'll also lose raw power as well, but with age comes knowing how to use remaining power to oh so much more effect.

I'm of the opinion that this should go under the Disgustingly Adorable pool.

No matter how you look at it, it's wrong.....Oh, guess it's not in Gensoukyou.